Search only text files with 'find' command?

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I've been using this to search an entire directory recursively for a specific phrase in my code (html, css, php, javascript, etc.):
Code:
find dir_name -type f -exec grep -l "phrase" {} \;
The problem is that it searches ALL files in the directory 'dir_name', even binary ones such as large JPEG images. With a directory also containing large number of images dispersed everywhere, it dramatically reduces the efficiency. Sometimes it takes very long to get a response.
Is there any parameter I can easily use to make it only search text files?
(HowTos)